Method for producing chromium oxide coated refractory fibers

ABSTRACT

A ceramic fiber and a shrink resistant article manufactured therefrom. The fiber comprises at least 80 weight percent of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, zirconia, zirconium silicate and mixtures thereof and which comprises less than 5 combined weight percent of alkali or alkaline metal oxide or alkali or alkaline metal silicate. The fiber is uniformly coated with from about 0.01 to about 5 weight percent of Cr 2  O 3 .

This is a continuation of copending application Ser. No. 041,033, filedMay 21, 1979, and now abandoned.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention concerns high temperature fibers and shrink resistantarticles manufactured therefrom.

(b) History of the Prior Art

In the prior art, amorphous or polycrystalline fibers were manufacturedeither from molten ceramic materials or from materials which wouldconvert to ceramic materials upon the application of sufficient heat.Examples of such fibers are mineral wool manufactured by blowing fibersfrom molten slag obtained as a by-product from metal refining, glasswool manufactured by blowing fibers from molten clay, silica andalumina; refined mineral fibers blown from molten iron silicates;ceramic fibers made from molten aluminum silicates; drawn glass fibersmanufactured from alkali metal silicates, alkaline earth metal silicatesand borosilicates; and spun fibers from inorganic or organiccompositions which convert to ceramic fibers upon the application ofheat such as solutions of aluminum chlorhydrate.

Of the foregoing fibers, aluminum silicate, silica and alumina-silicafibers are generally considered to have the highest temperatureresistance.

Such fibers, however, have disadvantages which have never beencompletely overcome. In particular, the heat resistance is still not ashigh as is desired and shrink resistance of fiber articles manufacturedfrom the fibers was not as good as desired.

Numerous approaches have been taken to improve heat resistance andshrink resistance in inorganic fibers at high temperatures. The mostcommon of such approaches has been to incorporate additives into thecomposition from which the fibers are made. Among such additives,chromium oxide has been used to increase temperature resistance. Forexample, U.S. Pat. Nos. 3,007,806 to Hartwig and 3,449,137 to Ekdahl;U.S. Pat. No. 4,125,406 to Sowman and British Patent No. 495,654 alldisclose that chromium oxide can be incorporated into a fusioncontaining silica and alumina followed by formation of fibers from thefused material.

While the incorporation of the chromium oxide into the fibers results infibers having higher temperature resistance and improvement in shrinkresistance of the fibers when formed into mats or other fiber articles,the process has serious disadvantages. In particular, oxygen is releasedby the chromium oxide in the melt which creates corrosion problems andresults in the presence of chromium metal which creates an unstablefiber product.

U.S. Pat. No. 3,019,117 to Labino discloses that a mass of glass fibersmay be dipped into a saturated solution of a metal nitrate such aschromium, iron, nickel or cobalt nitrates followed by drying the fibermass and subjecting the fiber mass to sufficient pressure andtemperature to fuse the fibers. While the resulting fiber block hasreasonably good temperature resistance, the flexibility and shrinkresistance at elevated temperatures is not as good as desired. This isbelieved, in accordance with the present invention, to be due tomigration of the solution prior to complete drying which results innon-uniform coating of the fibers. U.S. Pat. No. 2,839,424 to Labinodiscloses that fibers of an alkali silicate could be treated with anacidic salt such as an aqueous solution of calcium, zinc or bariumchloride so that the alkaline metal of the chloride replaces at least aportion of the alkali metal in the fiber followed by heating to driveoff water and treating the resulting fibers with a solution such aschromic anhydride to fill the resulting pores in the fiber with chromicoxide upon heating to a sufficient temperature. The resulting fiber ischaracterized by pockets of chromic oxide and according to the patent,is resistant to temperatures in excess of 2200° F. and, under certaincircumstances, when the fibers are completely covered, i.e., encased,with chromic oxide, will resist temperatures of about 3000° C. withoutdeformation. It is to be noted that the process for treating the fibersis complex utilizing at least two liquid treatment steps and numerousdrying and heating steps. It is to be further noted that the onlyspecific teachings with respect to a method for obtaining completecoverage of the fiber with chromium oxide is by incorporating the fiberinto a refractory brick structure containing between about 6 to about 16percent of temperature-resistant oxides such as chromium oxide by weightof product. Such a brick structure is not flexible. Additionally, thefibers discussed in U.S. Pat. No. 2,839,424 are fibers of an alkalisilicate which is a substance suitable for utilization in accordancewith the process of the Labino patent but which is highly undesirable ifresistance to humidity and chemical action is to be obtained.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the invention, there is provided a flexible shrinkresistant ceramic fiber article such as a fiber mat comprising ceramicfibers which comprise at least 80 and preferably at least 90 and mostpreferably at least 95 weight percent of a refractory compound selectedfrom the group consisting of silica, alumina, aluminum silicate,titania, titanium silicate, zirconia, zirconium silicate and mixturesthereof and which comprise less than 5 combined weight percent of analkali or alkaline metal oxide or alkali or alkaline metal silicate,said fibers being uniformly coated with from about 0.01 to about 5weight percent of Cr₂ O₃. Alkali metals include lithium, sodium andpotassium. Alkaline metals include barium, calcium and magnesium.

The invention also comprises the ceramic fiber uniformly coated withfrom about 0.01 to about 5 weight percent of Cr₂ O₃ from which theshrink resistant ceramic fiber article such as a mat is manufactured andthe method for uniformly coating a ceramic fiber comprising at least 80,preferably at least 90 and most preferably at least 95 weight percent ofa refractory compound selected from the group consisting of silica,alumina, aluminum silicate, titania, zirconia, zirconium silicate andmixtures thereof and which comprises less than 5 combined weight percentof an alkali or alkaline metal oxide or alkali or alkaline metalsilicate. The method of the invention comprises contacting the fiberwith a solution containing from about 0.1 to about 20 weight percent ofchromium ion; raising the pH of the solution to above about 9.5 whilethe fiber is in contact with the solution to precipitate a chromiumoxide hydrate upon the fiber; removing the fiber from the solution;vaporizing remaining free water from the fiber and heating the fiber tofrom about 200° to about 1200° C. to convert the chromium oxide hydrateto chromium oxide.

Another method contemplated by the invention for uniformly coating theceramic fibers with Cr₂ O₃ comprises the above method wherein the pH isnot upwardly adjusted but the solution contains urea or another compoundwhich thermally decomposes to produce ammonia or other basic compound.Alternatively, the pH of the chromium ion solution may be adjusted withammonium hydroxide or other basic compound so that the adjusted pH isslightly less than that required to precipitate the hydrous chromiumcompound. Upon exposure to heat, water evaporation produces a higher pHsolution in which the hydrous chromium compound precipitates.

Another method contemplated in accordance with the invention foruniformly coating a ceramic fiber with from about 0.01 to about 5 weightpercent of Cr₂ O₃ is to spray a solution of chromium ion on the ceramicfiber during the fiberization step and prior to initial cooling toambient temperature. The temperature at which the solution is applied issufficient to immediately vaporize the water and form a uniform chromiumcontaining compound on the fiber surface which calcines to form a Cr₂ O₃coated ceramic fiber.

DETAILED DESCRIPTION OF THE INVENTION

"Ceramic" means an inorganic compound having a decomposition orsoftening point above 1100° C. "Ceramic fibers", as used herein, meanssynthetic inorganic fibers which, both before and after treatment inaccordance with the present invention, have a decomposition temperatureor softening point greater than 1100° C. and preferably greater than1500° C. and a continuous maximum operating temperature greater than1000° C. and preferably greater than 1300° C.

The ceramic fibers with which this invention is concerned, are glass ormicrocrystalline fibers having diameters of less than 20 microns. Glassfibers are defined herein as fibers having no true crystallinestructure; i.e., amorphous fibers, and microcrystalline fibers aredefined herein as fibers comprising true crystals which, on the average,have a size of less than one-fourth of the diameter of the fiber.

Examples of inorganic compounds, from which ceramic fibers are made, inaccordance with this invention, are silicon dioxide (silica, SiO₂),aluminum silicate, aluminum oxide (alumina, Al₂ O₃), titanium oxide(titania, TiO₂), zirconium oxide (zirconia, ZrO₂), zirconium silicate,other high temperature oxides and silicates and mixtures and complexesthereof, and lower melting oxides and silicates such as iron oxides,feldspar and bentonite in low percentages; i.e., less than 10 percent,provided that less than 5 combined weight percent alkali and alkalinemetal oxides and silicates are used.

The most preferred fiber for use in accordance with the presentinvention is a fiber which contains from about 40 to 100, preferablyfrom 45 to 65 and most preferably 45 to 55 weight percent aluminum oxidewith between 0 and 60 and preferably from 35 to 55 weight percentsilica. Other compounds such as magnesia, iron oxide and ceramic oxidesand silicates may be used provided that greater than 5 weight percent ofcombined alkali and alkaline metal oxides and alkali and alkaline metalsilicates are not present since these compounds reduce maximum operatingtemperatures. Also, generally purer materials (e.g. purified silica andalumina) are frequently desirable. The most preferred fiber contains atleast 90% of combined Al₂ O₃ and SiO₂ which are usually at leastpartially combined with each other in the form of mullite.

In accordance with the invention, the fiber is uniformly coated withfrom about 0.01 to about 5 weight percent and usually 0.1 to 0.8 weightpercent of chromium oxide (chromia, Cr₂ O₃) which both raises heatresistance of the fiber and unexpectedly permits the formation of shrinkresistant mats, blankets and other particles to be formed from thefibers.

"Uniformly Coated", as used herein, means that chromium oxide on thecoated fibers is uniformly distributed along the length of the fiber inthe form of substantially equally spaced particles or crystals withoutchemically removing portions of the fiber surface.

In accordance with the invention, there are two major methods foraccomplishing a uniform coating of Cr₂ O₃ upon the fiber. The firstmethod comprises precipitating chromium oxide hydrate (Cr₂ O₃.xH₂ O)upon the fibers by means of a change in pH of a chromium ion solutionfollowed by conversion to Cr₂ O₃ and the second method comprisesspraying the fiber at an elevated temperature with an aqueous chromiumion solution to very rapidly vaporize the water, leaving a uniformdeposit of Cr₂ O₃.

In the first method, an aqueous solution containing from about 0.1 toabout 7 weight percent of chromium ion is treated with a base to raiseits pH to above about 9.5 in the presence of the fiber to be treatedthus precipitating chromic oxide hydrate upon the fiber. The chromiumion is provided by any suitable water soluble salt such as chromiumnitrate or chromium acetate. The base used to raise the pH may be anysuitable base such as ammonium hydroxide or sodium hydroxide or may be acompound which will thermally decompose to a compound which will providehydroxy ions upon heating of the solution. An example of such a compoundis urea which thermally decomposes to form NH₃ which in water forms NH₄OH.

Alternatively, the pH of the solution may be raised to a level below9.5, but above about 8 and the solution concentrated by evaporation thusraising the pH to above about 9.5 to precipitate Cr₂ O₃.xH₂ O upon thefibers.

After precipitation of Cr₂ O₃.xH₂ O the Cr₂ O₃.xH₂ O is converted to Cr₂O₃ by heating from about 200° C. to about 1200° C. for a sufficient timeto remove both free water and water of hydration leaving a uniformchromic oxide coating on the surface of the fiber. The time forconversion is dependent upon the conversion temperature which may befrom 0.5 to 2 hours at 200° C. and from 1 minute to one second at 1200°C. depending upon the quantity of free water on the fiber, the initialtemperature of the free water, the initial temperature of the fiber andCr₂ O₃, the quantity of Cr₂ O₃, the circulation and transfer of heat andthe number of fibers per unit heating area. The time usually is between2.5 and 35 minutes at about 700° C. depending upon mat thicknessesvarying between 0.5 and 5 centimeters and densities varying between 0.06and 0.13 grams per cc.

In the second method, a chromium ion solution similar to the solutionpreviously described, prior to precipitation of chromium oxide hydrate,is sprayed in the form of very fine droplets or mist upon the fiber;e.g., in droplet or particle diameter of from between molecular size toten times the diameter of the fiber. The droplets may be initiallysomewhat larger but become smaller as they approach the fiber due tovaporization. The temperature of the fiber struck by the droplets ormist is generally from the melting temperature of the fiber down to 350°C. and in the case of alumina-silica fibers when the solution is appliedat the time of blowing is usually from about 1500° C. to about 1750° C.

The fibers, uniformly coated with Cr₂ O₃ in accordance with theinvention, are characterized in having better heat resistance thanuncoated fibers or non-uniformly coated fibers of the same compositionand are characterized in that mats and other articles formed from thefibers have better shrink resistance at elevated temperatures than priorart mats or articles formed from uncoated or non-uniformly coated fibersotherwise having the same composition.

As used herein, elevated temperature means above about 0.8T where T isthe maximum continuous operating temperature in °C.

It has been further discovered that shrink resistance of articles madefrom the fibers can be additionally improved by pretreating the fibersat an elevated temperature in addition to coating them with chromiumoxide. The heating at an elevated temperature must be for aninsufficient time to cause the fibers to become brittle thus making themdifficult to handle. Generally the fiber is pretreated at a temperaturebetween about 1150° and about 1350° C. for from 1 to about 10 minutes.The higher pretreating temperatures require the shorter times.

Fiber articles or mats made from fibers of the invention are usuallymanufactured by compressing and needling the fibers to form a threedimensional structure which may be used as is or cut to a desired finalshape.

EXAMPLE 1

A mixture of 48 percent Bayer process alumina and 52 percent high puritysilica sand is melted in a furnace. A stream of the molten mixture atabout 1850° C. is then passed through an orifice at a rate of about 340kg. per hour. The molten stream is then first impinged by an air streamat sufficient pressure to turn the stream about 90°. The molten streamthen passes through a secondary stream of air which impinges the moltenstream at sufficient force and volume to form fibers having a diameterof about 3 microns. About 65 liters per hour of a liquid containing alubricant is applied to the stream at the primary nozzle to assistneedling.

The fibers are then collected on a moving conveyor mesh and compressedand needled to form a 2.5 cm. thick mat having a density of eight poundsper cubic foot (about 0.13 grams per cc). The mat is then heated tooxidize any remaining lubricant.

The resulting mat is then heated to about 1425° C. from ambienttemperature over a period of about 12 hours and held at that temperaturefor 7 and 14 days. The mat is found to have a linear shrinkage of 9.35%after 7 days and 10.44% after 14 days.

EXAMPLE 2

Example 1 is repeated except an aqueous solution, containing about 1.74weight percent chromium, provided by dissolving chromium acetate and 4weight percent lubricant, is substituted for the lubricant solution ofExample 1. The resulting mat after heating is uniformly coated with Cr₂O₃ and when tested is found to have a linear shrinkage of 7.48% after 7days and 9.04% after 14 days. The mat is found to have a chromium oxide(Cr₂ O₃) content of 0.47 weight percent.

EXAMPLE 3

Example 2 is repeated except prior to testing, the mat is prefired atabout 1200° C. for 5 minutes. The resulting mat retains flexibility andis found to have a linear shrinkage of 4.65% after 7 days and 5.72%after 14 days.

EXAMPLE 4

Example 2 is repeated except about 86 liters per hour of solution isapplied to the fiber. The resulting mat is found to have a Cr₂ O₃content of about 0.61 weight percent and a linear shrinkage of about6.61% after 7 days and 8.10% after 14 days at about 1425° C.

EXAMPLE 5

Example 4 is repeated except prior to testing for shrinkage, the mat isprefired at about 1200° C. for 5 minutes. The resulting mat retainsflexibility and is found to have a linear shrinkage of about 5.16% after7 days and about 6.51% after 14 days.

EXAMPLE 6

The process of Example 1 is followed except that the fibers are needledand compressed to form a 2.5 cm thick mat having a density of 6 poundsper cubic foot (about 0.1 grams per cc). The resulting mat is found tohave a linear shrinkage of about 6.65% after 9.5 hours at 1425° C.

EXAMPLE 7

The mat of Example 6 is saturated with a chromium acetate solutioncontaining 87 grams of chromium acetate per 1000 ml of water. Prior totreatment, the solution was adjusted to a pH of 9.5. The blanket isallowed to drain and is dried at about 170° C. which results in auniform precipitate. After heating at 1425° C. for 9.5 hours, the mat isfound to have a linear shrinkage of 5.82%.

EXAMPLE 8

Example 7 is repeated except the mat is prefired at 1200° C. for 5minutes prior to testing. The resulting mat retains flexibility andafter heating at 1425° C. for 9.5 hours, the mat is found to have alinear shrinkage of 2.39%.

EXAMPLE 9

Example 6 is repeated except kaolin clay fibers are used instead of thealumina and silica. The resulting fiber mat has a linear shrinkage of11.61% after heating at 1425° C. for 9.5 hours.

EXAMPLE 10

Example 7 is repeated except kaolin clay fibers are used. The resultingfiber mat has a linear shrinkage of 5.09% after heating at 1425° C. for9.5 hours.

EXAMPLE 11

Example 8 is repeated except kaolin clay fibers are used. The resultingfiber mat retains flexibility and has a linear shrinkage of 3.79%.

What is claimed is:
 1. A method for uniformly coating a ceramic fiber toincrease the shrink resistance of the ceramic fiber comprising sprayinga solution containing from about 0.1 to about 20 weight percent chromiumion upon the fiber at a fiber temperature of about 350° C. to themelting temperature of the fiber to form a fiber having increased shrinkresistance, said spraying occurring during fiberization prior to initialcooling.
 2. The method of claim 1 wherein the fiber is formed from acomposition comprising at least 90 weight percent of a mixture ofalumina and silica and the fiber temperature at the time of spraying isfrom about 1500° to about 1750° C.
 3. The method of claim 2 wherein thefibers comprise from about 40 to 100 weight percent alumina and from 0to 60 weight percent silica.
 4. The method of claim 3 wherein the fiberscomprise from about 45 to about 65 weight percent alumina and from about35 to about 55 weight percent silica.
 5. The method of claim 3 whereinthe fibers comprise from about 45 to about 55 weight percent alumina andfrom about 45 to about 55 weight percent silica.